Why is Kate Middleton hospitalized?

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(CNN) — Whenever there’s a royal wedding, waiting for the news that a royal heir is on the way is always the next step. Less than two years after Britain’s Prince William wed his bride Catherine Middleton, word of a royal pregnancy was eagerly anticipated. Now we know Catherine is having a baby.

But the world didn’t find out in the form of a photo revealing a conspicuous baby bump. Rather, the news broke when Buckingham Palace announced Monday the Duchess of Cambridge has been “admitted … to King Edward VII Hospital in Central London with hyperemesis gravidarum,” – which means excessive vomiting during pregnancy.

As most mothers can attest, feeling nauseated during pregnancy is not unusual, so why is the duchess hospitalized?

There are no details from the royal family, but hyperemesis gravidarum is a fairly common condition for women who are sensitive to the higher hormone levels that come with pregnancy. Each month during a woman’s menstrual cycle, the estrogen circulating through her body rises, says Dr. James Liu, who is the chairman of the Department of Obstetrics and Gynecology at University Hospital’s MacDonald Women’s Hospital in Cleveland.

“But once a woman achieves pregnancy the ovary at the beginning and later on the placenta makes much higher levels of estrogen and progesterone; and very high levels of estrogen in general can have an effect on a part of the brain called the chemoreceptor zone and it causes nausea.” During pregnancy, the hormone levels can rise 100 to 1,000 times the amount of a normal menstrual cycle.

Extreme nausea and vomiting during pregnancy may occur if a woman is carrying more than one baby, according to the U.S. National Institutes of Health. Another possible cause for higher hormone levels leading to excessive nausea is something called abnormal placentation, where the placenta overgrows, says Liu, adding that this happens only occasionally.

Usually, excessive vomiting is treated on an outpatient basis, where the mom-to-be is given anti-nausea medication, usually in pill form. But vomiting can lead to dehydration – a concern for a pregnant woman.

If she is very dehydrated, being in the hospital allows her to get fluids intravenously. Her electrolytes or blood minerals may also be somewhat abnormal, which can be corrected through an IV too.

If she’s been vomiting a lot, she probably hasn’t been able to keep oral anti-nausea medication down – another reason she may be hospitalized and receive IV medication, says Liu, who is not involved in the duchess’ care.

The severity of nausea during pregnancy varies from woman to woman, Liu says. Some may experience nausea but may not vomit, while others may be very sensitive, vomit excessively and are unable to keep fluids down.

The good news is that the nausea usually goes away by the end of the first trimester, says Liu. “The first 12 weeks of the pregnancy is the most common time we see this.”